In telephone systems, trunk circuits serve as an interface between communication or, more particularly, transmission lines connected to a distant switching office and the switching network of a local telephone switching office. The establishing of a transmission path between adjacent offices is usually preceded by certain signaling between the offices. Commonly, the lines are designed to assume one of two impedances for the purpose of such signaling. Generally, a trunk circuit includes a power supplying circuit and a signaling detector circuit to detect the high and low impedances of the line.
Since the current in a line is a function of the impedance of the line, the two impedances may be detected by any one of a number of well-known current detectors such as, for example, relays and the like. Characteristically, these current detectors have one threshold level against which current applied to the detector is compared. Current above the threshold level causes the detector to generate an output signal indicative of one impedance of the line. Current below the threshold level causes the detector to generate another output signal indicative of the second impedance of the line. Thus, it is common practice to select a current detector having a threshold level somewhere in between the two current levels which are a function of the two impedances of the line.
However, with more complicated signaling schemes, communication lines are often designed to assume one of three, four, or more impedances, different pairs of impedances from the plurality commonly being utilized for each operative mode of the line. In the prior art, relays and the like are switched in and out of the line to detect two current levels for each operative mode of the line. Since these detector circuits are costly and usually bulky, it would be desirable to have one detector circuit that could detect the different pairs of impedances presented by the line for each operative mode.